Angles on opposite sides of the point where lines cross are called "vertical angles." (The same lines define both angles, but they have only the vertex in common.) Their measure is irrelevant. The important characteristic of vertical angles is that they are equal in measure.
Since the angles have equal measures, we can equate the expressions for them:
3x +20 = 10x -15
35 = 7x . . . . . . . . . add 15-7x to both sides of the equation
